# /DT/FVMBAG/Iflag

Engine Keyword Time step for control for Finite Volume Method airbag.

## Format

• For Iflag = 0, 1

/DT/FVMBAG/Iflag

$\text{Δ}{T}_{sca}$ $\text{Δ}{T}_{\mathrm{min}}$

• For Iflag = 2

/DT/FVMBAG/Iflag

$\text{Δ}{T}_{sca}$ $\text{Δ}{T}_{\mathrm{min}}$ Lambda L_type

Monvol_ID

## Definition

Field Contents SI Unit Example
Iflag Finite volume merge flag.
= 0 or blank (Default)
The finite volume is merged, if the time step < airbag volume < 10 * the minimum volume defined in global merge, /FVMBAG/MODIF.
= 1
The finite volume is merged, if time step < $\text{Δ}{T}_{\mathrm{min}}$ .
= 2
The finite volume is merged, if time step < $\text{Δ}{T}_{\mathrm{min}}$ . The time step computation is defined by the flag L_type. 2

$\text{Δ}{T}_{sca}$ Scale factor on time step.

Default = 0.9 (Real)

$\text{Δ}{T}_{\mathrm{min}}$ Minimum time step.

(Real)

Lambda Stability factor.

Default = 0.01 (Real)

L_type Time step computation method.
= 1
Characteristic length of FVM mesh (computed during Starter).
= 2 (Default)
Characteristic length is computed from cubic root of the volume.

Monvol_ID Monitored volume identifier.

(Integer)

1. The $\text{Δ}{T}_{sca}$ and $\text{Δ}{T}_{\mathrm{min}}$ can be defined to ensure stability and a minimum time step size by triggering additional finite volume merges when the FVMBAG entity penalizes the computation.
2. When using /DT/FVMBAG/2, time step is computed as:(1)
$\text{Δ}T=\text{Δ}{T}_{scal}\frac{\text{Δ}x}{\left|u-w\right|+c}$
Where,
$u$
Gas velocity.
$w$
Polyhedron velocity.
$c$
Speed of sound.
$\text{Δ}x$
Characteristic length.

The soothing factor is applied using $\text{Δ}T←\mathrm{min}\left(\text{Δ}T,\text{Δ}{T}_{old}\left(1+\lambda \right)\right)$ .